Chronic inflammation is closely associated with carcinogenesis. One potential mechanism for inflammation-induced carcinogenesis involves DNA damage and mutation caused by reactive halogen species that are produced by myeloperoxidase to kill pathogens. The major DNA lesions formed by reactive halogen species include 8-halogenated guanine (haloG) such as 8- chloroguanine and 8-bromoguanine. HaloG is a promutagenic lesion that can trigger misincorporation of G opposite the lesion, promoting G to C mutation. Recently, haloG excision activity was observed in rats, yet an enzyme responsible for such activity has not been identified. Our preliminary study showed that human 8-oxoguanine glycosylase hOGG1 efficiently cleaves haloG from DNA, suggesting hOGG1 as a repair enzyme for haloG. Our central hypothesis of the proposed research is that haloG is a promutagenic lesion that affects various biological processes, and repaired by base excision DNA repair. Our long-term goal of the proposed programs is to elucidate the effects of guanine 8-modification on biological processes such as DNA repair, DNA replication, transcription, DNA methylation, and tumorigenesis. The objectives here are to elucidate mechanisms of haloG repair and haloG- induced mutagenesis and to evaluate the effects of haloG on epigenetic mechanisms. As a next step for achieving our long-term goals, we have designed three Specific Aims that are 1) Elucidating haloG recognition and repair mechanisms of hOGG1; 2) Clarifying structural basis for haloG-mediated mutagenesis; and 3) Evaluating the effects of haloG in CpG dinucleotides on epigenetic mechanisms. Our expectation is that the successful execution of these programs would advance our understanding on the inflammation-induced DNA damage and repair and the effects of inflammation-induced lesion on epigenetic mechanisms, providing important insights into the role of chronic inflammation in cancer etiology.